The long-term goal of these experiments is to define the cellular and molecular mechanisms responsible for morphological transitions that occur during cycle 14 of the Drosophila embryogenesis. Over the next five years we will focus on a subset of those phenomena that provide a bridge between cellularization and the cell shape changes of gastrulation. The first set of experiments addresses the formation of the basal and apical junctions during cellularization and the role played by the nullo gene in that process. Junction formation will be studied in living embryos and fixed material and genetic screens will be carried out to identify new components in the process. The second set of experiments examines the link between certain cellularization phenomena and the cell shape changes that occur in the ventral furrow at gastrulation. Our analysis will focus on apical flattening and constriction, processes that involve a re-distribution of myosin from the cellularization front to the apical surface. We will utilize controlled expression of the folded gastrulation gene and disruption of the basal and apical junction components to investigate the importance of the linkage between the contractile cortex and the incipient zonula adherens. The third set of experiments focuses on the initiator cells of cephalic furrow formation and the relationship between their behavior and a newly discovered spatial pattern in the diameters of stalk constrictions during cellularization. We will investigate the mechanism that widens the cleavage furrow canal and will characterize the defects associated with s locus in 64BC that affects both gastrulation and cellularization. We will clone the gene responsible for those effects and test for interactions with other genes involved in furrow widening and the control of stalk diameter.